The invention relates to a convergence and geometry adjustment device for a video-projector comprising several cathode ray tubes.
A video-projector is a television receiver projecting images on a screen of a larger size than the usual screens of cathode ray tubes. The most widely used type comprises three cathode ray tubes, one for each fundamental color and, for each of these tubes, a lens for projecting on a screen.
A video-projector usually comprises geometry correction circuits acting on the scanning of the three tubes for correcting the usual deformations of the television image such as north-south pincushion and east-west pincushion, as well as deformation specific to the video-projector, which are due to the variable slant of the axes of the tube with respect to a perpendicular to the screen. In fact, the most usual case is a vertical screen and tubes whose axes are not in a horizontal plane, but in a plane slanting upwards in the projection direction; the shape of the projection surface also forms a possible cause of geometric deformation of the image.
These defects of geometry are corrected by acting on the horizontal and vertical deflection fields using geometry correction circuits. Some geometry corrections are independent of the orientation of the tubes with respect to the screen or of the shape thereof; they are generally provided by the manufacturer. Other connections are made by the user (or the installer): these corrections are for horizontal trapezoid, vertical linearity and vertical amplitude faults. A vertical amplitude fault is a deviation of the height of the image with respect to the normal; a vertical linearity fault is the non-conservation of distances in vertical direction and a horizontal trapezoid fault is a deformation of the image which, instead of being rectangular, has the form of a trapezoid with parallel horizontal edges. These corrections may be made by adjusting the position of the screen with respect to that of the video-projector. This operation is relatively difficult to carry out. This is why these corrections are often made by correcting or modifying the scanning action; in known devices, three adjustments are provided on the apparatus: the first for the amplitude, the second for the linearity and the third for the trapezoid defect. These adjustments are also difficult to use.
The invention overcomes this drawback.